1. Field of the Invention
This invention relates to a cooled turbine blade and more particularly to a water-cooled blade.
2. Description of the Prior Art
Water-cooled gas turbine rotor blades are well known in the art as typified by U.S. Pat. Nos. 3,804,551 and 3,736,071 in which the water enters the blade adjacent the blade root and flows in a generally radially outwardly direction through cooling passages subadjacent the skin of the blade to ultimately be exhausted into the motive gas stream of the gas turbine, substantially in vapor form. However, in such an enviroment, because of the large centrifugal force field and because of the difference in densities of the fluid, the radially outward flow of the cooler water tends to overtake the outward flow of the vapor, causing vapor blockage of the coolant flow through the passages, thereby reducing the heat flux capability of the passages and ultimately causing the blade to fail from overheating. Also, fluid flow through a cooling channel is inherently unstable when boiling occurs therein if the flow direction coincides with the gravitational or force field. This is particularly critical in parallel channels having a common reservoir wherein such flow can cause one channel to become substantially filled with liquid, which because of the then increased density of the fluid in this channel, causes more liquid flow into this channel which may starve the flow to the other channels, causing overheating in their vicinity.
One attempt to overcome the above deficiencies is illustrated in U.S. Pat. No. 3,902,819 wherein the water flowing through the coolant passages is maintained at a supercritical pressure so that it cannot vaporize and cause the blockage or unstable flow. However, this reduces substantially the amount of heat that can be absorbed (i.e., the heat for vaporization being a considerable portion of the cooling capability of the previous referenced blade configurations) and requires any make-up water to be introduced at the supercritical pressure in the system.
Another approach to the problem of eliminating steam pockets, from the coolant passages to enhance the heat transfer is shown in copending commonly assigned patent application Ser. No. 773,461 wherein intermediate enlarged chambers are provided in the coolant passages to permit the heated coolant (water) to flash to steam, with the steam being vented in a separate path from the coolant flow which continues to an exhaust port adjacent the blade tip. However, ultimately, it is expected that water will vaporize prior to being exhausted at the tip.